474 



LIEBEBMANN 



[chap. 11 



5. Natural Electromagnetic Radiation or "Noise" in the Sea 



The upper Hmit of the natural electromagnetic noise spectrum in the sea is 

 obviously limited by the attenuation of electromagnetic radiation in sea-water. 

 It was shown above that the possibilities of penetration above 100 c/s are poor, 

 and this frequency shall be arbitrarily taken as the upper limit of electro- 

 magnetic noise. 



In the frequency range 5 c/s to 100 c/s world-wide lightning storms provide 

 the dominant source of electromagnetic noise. The level of noise arising from 

 this source is surprisingly constant. This is because there are always more than 

 100 lightning strokes per second averaged over the whole world, day or night. 

 Because electromagnetic radiation in this frequency range propagates with low 

 attenuation, distant lightning storms, originating over a large fraction of the 

 earth, generate a rather constant output of noise. 



> CP 





001 



0.1 



10 



100 



FREQUENCY (c/s) 

 Fig. 2. Spectrum level of natural electromagnetic noise. 



Below 5 c/s lightning ceases to make an important contribution to the noise. 

 The origin of this noise in the band 5 c/s to 0.001 c/s remains unknown, but 

 correlation with sun-spot activity is sometimes established. In addition to a 

 general background level, there are at least two types of sinusoidal damped 

 oscillations which may persist for many cycles and also impulsive noise signals 

 of shorter duration. These signals are generally intermittent occupying only a 

 fraction of the measurement time. 



Fig. 2 represents a compilation of data on the spectrum of natural geo- 

 magnetic noise. It is seen that the spectrum level diminishes with increasing 

 frequency approximately according to the relation /~i- 2. Virtually all the data 

 given are for the high-amplitude, highly intermittent noise. Almost nothing is 

 known about the steady noise background. The data are predominantly for 

 the horizontal component. The conductivity of sea-water tends to damp the 

 vertical component of the magnetic field so that only the horizontal component 

 is of major importance within the ocean. 



The amplitude is strongly latitude dependent, increasing as the auroral zone 

 is approached. However, in the northern latitudes, where the total field direc- 

 tion is nearly vertical, the noise is much diminished by the effect of induced 



